The present invention relates to a method of analysing solutions or dispersions of dyes or their precursors and intermediates by an optical and/or spectroscopic technique, which method comprises using apparatus based on the principle of attenuated total reflection as sensor system.
In recent years, increasing efforts have been made to automate dye manufacture, both with respect to the actual preparatory process itself and to the working up. To obtain satisfactory and reproducible results it is necessary to rely on analytical methods which are characterised by the following criteria: short duration of analysis, high frequency of analysis, low costs, simplicity, reliability and space requirement. Apparatus that meets these requirements consists in particular of sensors for pressure, temperature and pH measurement. Also important, especially in the dyestuffs sector, is the recording of optical data preferably in a wide wavelength range. Spectroscopic methods have attained importance with the development of fibre optical waveguides, as measuring apparatus and measuring cell can now be installed separately. The advantages of spectroscopic methods reside in their high selectivity, thereby also making possible the rapid analysis of complex organic mixtures without complicated separation procedures. Further, this method of analysis ideally meets the requirements mentioned at the outset.
Important for making optical measurements in the visible or also infrared and ultraviolet range is suitable apparatus for passing the beam of light used for the analysis through the liquid medium. The simplest piece of equipment is a flow cell which, however, usually requires a by-pass through which the medium to be analysed is continually circulated. In addition, particles on which the light is scattered prove exceedingly troublesome in this method. Aside from the flow cell, other systems which are solely immersed in the medium to be analysed have meanwhile been developed. These systems have become generally known as optical sensors (Optrodes.RTM.). Optical sensors for measuring transmission consist e.g. of a two-way bundle of fibres and a mirror. However, for measuring stray light and reflectivity, similar constructions without mirror are employed. The most serious drawback of such optical sensors and of the flow cell is, however, that concentrated solutions or solutions with a solids content cannot, or can no longer, be analysed accurately and reproducibly. But the fact is that strongly absorbing and highly concentrated solutions are the rule in dyestuff chemistry. Analysis of such solutions is therefore always preceded by a complicated sampling involving repeated dilution and/or separation of undissolved components. This sampling is time-consuming and errors are inherent in it. Continuous monitoring of a process in this manner is virtually impossible.